feat(inbound-filters): Gate custom filter writes on the plan feature

[shellmayr]

• Custom inbound filters are a plan feature (projects:custom-inbound-filters, Business plans). The API refuses writes without it, but the UI did not say so before the save failed.
• The "Add Inbound Filter" item in the issue More Actions menu now shows disabled with "Your plan does not include custom inbound filters" on such projects, like the discard item does for its plan feature.
• The custom filters table in project settings disables Add Filter, the active toggle, edit, and delete the same way, with the reason as tooltip. Reads stay, so an organization that left the plan still sees its filters.
• Based on feat(inbound-filters): Add an issue action that drafts a custom filter from the event #126853 and lands after it.